Phase modulation receiver



July 9, 1940. c. w. HANsl-:LL

PHASE MODULATION RECEIVER Filed Jan. 18, 1939 INVENTOR.

n. N T T A E we@ E D A a M Patented July 9, 1940 UNITED STATES PHASE MODULATION RECEIVER Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 18,

6 Claims.

This invention involves phase modulation reception in a novel manner by a novel means. By this new means I avoid many of the difliculties which are met with in some of the phase modulation methods and receivers known in the past. In one known type of phase modulation receiver the carrier wave of the received signal is separated out by means of crystal filters or' other highly selective circuits, increased in relative amplitude, changed in phase by 90 degrees and reintroduced with the modulation side frequencies in a manner to convert a phase modulated Wave intoI an amplitude modulated wave. Other known types of phase modulation receivers use as a component part selective lters or circuits in various manners. In these known receivers the necessity of extremely accurate control of the carrier frequency is encountered, usually at an intermediate frequency following a first heterodyning operation, in order that the carrier frequency may be held within the -pass band of the crystal filters or highly selective circuits. Moreover, in some methods used in the past it has been found desirable tohold down the degree of phase modulation at the transmitter to such an extent that second, third and higher order side frequenicies may not become of appreciable amplitude. If the higher order side frequencies do become appreciable in amplitude they cause harmonic distortion of the detected modulation, when the wave is converted to amplitude modulation and then rectified or detected.

My present invention involves conversion of phase modulation into frequency modulation, 35 before final detection, in a superheterodyne type of receiver having a nal detector designed to demodulate a frequency modulated wave. I convert a phase modulated wave into a frequency modulated wave, in the receiver, by means of modulation frequency energy which is derived from the receiver circuits, after the frequency modulation detector, and fed back to frequency modulate a heterodyne or beating oscillator which is cooperating in the production of the frequency modulated energy fed to the frequency modulation detector. The potentials fed back are modified in a manner such that the frequency of the beating oscillator is modulated unequally for the various modulating frequency energies. The non-uniformity of po-tentials fed back is such as to result in conversion of the received phase modulated wave into intermediate frequency energy which is frequency modulated in a manner characteristic of the modulations on the phase modulated Wave being received.

1939, Serial N0. 251,473

(Cl. Z50-20) To carry out my invention I may use an assemblage of parts already well known in the art for carrying out certain well known functions. It is, therefore, not believed to be necessary to describe each of these parts in detail for enabling one skilled in the art to practice the invention.

In describing my invention reference will be made to the attached drawing wherein Figures l and 2 each show diagrammatically the essential elements comprised in two modiiications of my novel receiver. Other modifications will be apparent to' those skilled in the art and for that reason, my invention is not to be considered to be limited except as deiined in the claims.

One form of the invention is shown in Figure l, in which I have illustrated diagrammatically a receiving antenna A or line L delivering energy characteristic of a phase modulated transmitted wave to the receiver. The received energy may be selectively amplified in amplifier I 0 and is then beat down to a lower intermediate frequency in a heterodyne detector or converter also in I0 by cooperating, or beating, of the received energy with energy derived from a heterodyne oscillator I2 included in the receiver.

The intermediate frequency energy is then selectively ampliiied'in I4 and applied to a frequency modulation detector I 6 of any known suitable type. I` prefer a frequency modulation detector of the balanced, or push-pull type,` which treats corresponding side frequency energies, symmetrically located around the carrier frequency, equally in the detection process. The detector output, which contains modulation frequency energies corresponding to' those applied to the distant phase modulated transmitter, is then amplified in I8 and passed on to any desired utilization device such as a loudspeaker, telegraph line, tone keyer, recorder, repeater station, etc.

v At some point after the frequency modulation detector a small portion of modulation frequency current is taken out through resistances and passed through an inductance 22. 'I'he current will cause a voltage or alternating potential to appear across the inductance 22 which has a value substantially proportional to the modulation potential at each frequency in the output of the frequency modulation detector multiplied by the modulation frequency. The alternating potential developed across inductance 22 is utilized to cause a controllable frequency modulation of the first heterodyne oscillator I 2. The amount of this modulation, or the frequency deviation of the oscillator, for any given modulation frequency power in amplifier I8 is almost proportional to the modulation frequency and is given such polarity as to cause the degree of phase modulation of the carrier wave, at the intermediate frequency, to be reduced substantially in proportion to the inverse of the modulation frequency. As a result the phase modulated wave which would otherwise have been delivered to the frequency modulation detector is converted into a frequency modulated wave having a lower phase or frequency deviation than the phase or frequency modulation for each component frequency in the original phase modulation.

The overall result is that the frequency modulation detector I6 delivers an output which is sub stantially a correct reproduction of the original modulation input to the distant phase modulated transmitter. Thus I am able to use a frequency modulation detector to demodulate a phase modulated wave, without distorting the frequency characteristic of the modulation.

The modulation reducing arrangement of Figure l is useful in that it reduces the average strength of side frequency currents, as compared with carrier currents, and gives an equivalent eX- altation of the carrier before detection. This is useful in reducing distortions brought about by selective fading which, as is well known, can cause over modulation of the carrier and distortion of the received modulation when the fading reduces the strength of the carrier to less than the peak value of the combined energies of the other frequency components present with the carrier. This process of carrier exaltation does not increase the effect of noise with respect to the signal because noise frequency components are reduced in the same proportion as the signal side band power. As a matter of fact the effect of noise is reduced of the carrier exaltation, as will be understood by those who have studied the advantages which result from exalting the carrier wave, with respect to the side band power, at the receiver.

In the arrangement of Figure 1, if the resistances 29 are connected directly to the frequency modulation detector output so that steady state unbalance in detector currents will be applied to the resistances 2D and if the inductance 22 also has resistance, or if resistance is used which becomes effective at zero and very low frequencies, it is possible to obtain automatic tuning or frequency control by varying the frequency of the first beating oscillator to keep the intermediate frequency always at almost the exactly correct value. That is, if 22 has sufficient resistance or resistance is inserted in this circuit a direct current potential may be developed therein which is a function of the mean frequency of the intermediate frequency, and this potential may be used to regulate the mean frequency of operation of the oscillator in l2 to hold the mean I. F. frequency substantially free of slow variations. Although it is usually preferable to derive the frequency controlr energy for automatic tuning from the frequency modulation detector vacuum tubes directly this is not necessary if the amplifier I8 is designed to act as a zero frequency or direct current amplifier as well as a modulation fre quency amplifier. The automatic tuning feature will be rendered more `effective if the resistances 20 are each shunted by an inductance which will pass direct currents freely, but which will pass only inconsequential amounts of modulation frequency ciurents.

i It may be well to bring to` attention the possi bilities for instability in the receiver of Fig. 2 brought about by reversal of phase of some frequencies fed back outside the normal useful frequency band. This instability may call for application of some of the inventions already known for eliminating such instability in inverse feed back circuits generally.

The receiver of Figure 2 is similar in effect to that shown in Figure 1 inthat it converts a phase modulated wave into a frequency modulated wave before nal detection. However, in the receiver of Figure 2, the potential for modulating the first beating oscillator is derived by the passing of modulating frequency currents resulting from demodulation through resistance 2l and capacity 23 in series. The alternating current component of potential across the capacity 23 is then used to frequency modulate the first beating oscillator l2 in a direction to increase the phase deviation of the modulated current in the output of the heterodyne first detector lll. This results in converting the original phase modulation into frequency modulation before final detection. At the highest modulation frequencies the increase in phase deviation at the intermediate frequency may be small, but the increase will become greater at lower and lower modulating frequencies within the useful range of modulating frequencies.

In the arrangement of Figure 2, it is desirable that direct current components of modulation be eliminated from the energy fed back to frequency modulate the first beating oscillator. This is accomplished by means of an alternating current transformer anywhere between the frequency modulation detector and the first beating oscillator. Direct current components are not passed on by the transformers which may be located as shown between 23 and the oscillator l2.

Of course, if desired, automatic frequency or tuning control may be utilized in the receiver of Figure 2 by deriving direct current potentials from the frequency modulation detector I6, or amplifier i8, and employing them with suitable polarity to control the average frequency of oscillator l2. Circuits for carrying this out have not been indicated in the diagram, but one skilled in the art can readily supply them.

Having thus described my invention, what I claim is:

l. In a system for demodulating phase modulated wave energy, a detector system normally responsive to frequency modulated waves, means for impressing phase modulated waves on said detector system, means for modifying modulation potentials derived from said detector system in accordance with their frequency, and means for modifying the phase modulations on said Wave energy in accordance with said modified potentials.

2. A system as recited in claim l wherein said second named means is inductive in character and modifies said modulation potentials substantially in proportion to their frequency.

3. A system as recited in claim l wherein said second named means is capacitive in character and modifies said modulation potentials substantially inversely in accordance with their frequgncy.

4. The method of demodulating phase modulated wave energy which includes the steps of, beating said wave energy with oscillations to obtain wave energy of lower frequency, demodulating said wave energy of lower frequency to derive signal potentials which are not a true repn resentation of the Wave modulations, and modulating said oscillations by means of said potentials in a manner to cause said signal potentials to be a true representation of said Wave modulations.

5. In a system for demodulating phase modulated Wave energy, a demodulated system normally responsive to frequency modulated wave energy, phase modulated wave amplifying means including anoscillation generator and a heterodyne detector, means for impressing said phase modulated Wave energy and oscillations from said oscillation generator on said heterodyne detector, means for impressing the beat frequency output of said heterodyne detector on said demodulation system, means for modifying modulation frequency potentials derived from said demodulation system in4 accordance with their frequency, and means for modifying the Wave length of the oscillations produced by said generator in accordance With said modified potentials to thereby modify the modulations on the beat frequency output of said heterodyne detector.

6. A system as recited in claim 5 wherein means is included with said last named means to control the mean frequency of operation of said oscillation generator in accordance with slow changes in the mean frequency of said beat frequency output of said heterodyne detector.

CLARENCE W. HANSELL. 

